This ticket is to track a cleanup from LU-11566.

https://review.whamcloud.com/#/c/34523/

Description:

LU-11566 llog: consolidate argument validation

Add a helper function to validate the start/end arguments passed from
userspace for "lctl llog_cancel" and "lctl llog_print" to reduce code
duplication.

The llog callback data is static in these functions, for use by repeat
calls for each record in the log. This is not really safe, if there
are multiple threads calling the llog_print or llog_check functions
concurrently. Fortunately, this is unlikely to be the case, but it
would be better to allocate them dynamically in the caller.